tab changes at checkout

I'm trying to move to the LG X Power 2, which is $150 off, making it $90 on my tab or $4/month on tab small.

When I go to my cart it says Tab Small $4 a month after the $150 discount. But when I go to finish the order it changes to $10 per month tab. Am I missing something? Or is the tab just going to paid off in half the time?

I'm having the same issue trying to get the Galaxy S7. Says it's a $9/month charge until I input my c/c info where it then changes to $10/month. It's not a big change but I'd like to know why it's changing on checkout.